1. Field of the Invention
This invention relates to a method for breeding domesticated fowl for increased disease resistance. In a particular aspect, the invention relates to a method for breeding chickens raised for meat and eggs to achieve increased disease resistance. In a further related aspect, the invention relates to a method for determining the Rfp-Y of B-F haplotype of domesticated fowl, including chickens.
2. Description of the Background Art
In domesticated fowl, the major histocompatibility complex ("Mhc") which is associated with the regulation of immune recognition and immune response, is called the B system. This system, which comprises polymorphic Mhc class I, class II and B-G genes, has been known to exist since the early 1940's. Briles, W. E. et al., Genetics, 35:633-652 (1950), Pink, J. R. L. et al., Immunogenetics, 5:203 (1977). U.S. Pat. No. 5,451,670 to Miller, M. M. et al. describes a procedure for determining the genotype of the B-G region of the Mhc. Cosmid cluster I on the molecular map of the chicken Mhc genes published by Guillemot, F. et al., EMBOJ, 7:2775-85 (1988) corresponds to the B system. Resistance to Marek's disease and other diseases, general fitness and productivity have been associated with the B system haplotype.
Genotyping birds for the B system of histocompatibility can be accomplished by five different kinds of tests. The first, and by far the most commonly used method, is a serological test: hemagglutination of chicken red blood cells with alloantisera. This method requires some prior knowledge of the genetics of the animals and availability of appropriate alloantisera. The second relies on the patterns of B-G gene restriction fragments revealed in genomic DNA digested with a restriction enzyme and analyzed in Southern hybridization with nucleic acid probes for the B-G genes. See Miller, M. M., U.S. Pat. No. 5,451,670. An advantage of this approach is that prior knowledge of gene sequences is not necessary. A third method relies on B-F (class I) and B-L (class II) gene restriction fragment patterns revealed in genomic DNA digested with several restriction enzymes and analyzed by Southern hybridization with nucleic acid probes for the B-F and B-L genes. See Lamont, S. J. et al., Poult. Sci., 69:1195 (1990). A fourth method is based on hybridization of oligonucleotide probes specific for known sequences in the various alleles of the B system class I gene (gene B-FIV on the physical map of chicken Mhc genes (See Guillemot, F. et al., 1988, supra.)). This method requires knowledge of the sequence of the allele at least in the region to which the probe hybridizes. See Shuman, R. M. et al., "Development of an Mhc Typing Test Using DNA Amplification and Oligonuleotide Probes", Poult. Sci., 72 (Suppl. 1): 10 (Abstr.) (1993). A fifth method employs antibodies developed to a specific epitope on class I antigens through expression of recombinant genes in chickens.
The use of a technique known as polymerase chain reaction, single-stranded conformational polymorphism ("PCR-SSCP") to study the expression of B-G genes in non-erythroid tissues has been proposed. Miller, M. M. and Goto, R. M., Avian Immunology in Progress, Tours (France), Aug. 31-Sep. 2, 1993, Ed. INRA, Paris 1993 (Les Colloques, No. 62). In this method, short segments of B-G genes of interest are amplified using PCR. The PCR products are then denatured by heating and applied to a non-denaturing polyacrylamide gel. The single-stranded fragments of the heat-denatured DNA fragments assume secondary conformations determined by their sequences and migrate differently in the polyacrylamide gel during electrophoresis, so as to produce a pattern (or fingerprint) representative of the sequences within the genome in the region of amplification.
Recently, a second system of major histocompatibility genes of the chicken has been discovered. Briles, W. E. et al., Immunogenetics, 37:408-414 (1993). This system, designated Rfp-Y, consists of at least two class I genes, three class II genes and a c-type lectin gene. Miller, M. M. et al., Proc. Nat'l. Acad. Sci. USA, 91:4397-4401 (1994). Haplotypes of Rfp-Y assort independently from haplotypes of the B system. Briles, W. E. et al. (1993), supra.
The existence of a second genetically-independent complex of polymorphic histocompatibility genes was unexpected, because Mhc genes are typically considered to be in a single linkage group. Previous studies have suggested that at least one Mhc class II gene in the B-L .beta.III gene family, now known to be within Rfp-Y is expressed. Zoorob, R. et al., Eur. J. Immunol., 23:1139-45 (1993). Transcripts of an Mhc class I gene within Rfp-Y were also found in many different tissues. See Afanassiff, M. et al., Abstract presented at the Avian Immunology Research Group Meeting, Obergurgal, Austria, Apr. 21-24, 1996. Nevertheless, the role of the Rfp-Y system in immune mediation of disease resistance and the extent to which genes of the Rfp-Y system are expressed in various cell types has heretofore remained unknown.